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Abstract

Spatial positioning of nanocrystal building blocks on a solid surface is a
prerequisite for assembling individual nanoparticles into functional devices. Here,
we report on the graphoepitaxial liquid-solid growth of nanowires of the
photovoltaic compound CH3NH3PbI3 in open
nanofluidic channels. The guided growth, visualized in real-time with a simple
optical microscope, undergoes through a metastable solvatomorph formation in polar
aprotic solvents. The presently discovered crystallization leads to the fabrication
of mm2-sized surfaces composed of perovskite nanowires having
controlled sizes, cross-sectional shapes, aspect ratios and orientation which have
not been achieved thus far by other deposition methods. The automation of this
general strategy paves the way towards fabrication of wafer-scale perovskite
nanowire thin films well-suited for various optoelectronic devices, e.g. solar
cells, lasers, light-emitting diodes and photodetectors.

Introduction

One-dimensional nanostructures, such as nanowires and nanotubes, represent the smallest
dimension for efficient transport of electrons and excitons1. In
particular, organic or solid-state nanowires, due to their unique properties as compared
to their bulk counterparts, are attractive building blocks in a wide range of
applications, including sensors2,3, nanoelectronics4,5,
photonics6,7, and renewable energy8. Recently, a
solvatomorph-mediated synthesis of hybrid halide perovskite in nanowire form,
specifically methylammonium lead iodide, CH3NH3PbI3
(hereafter MAPbI3), a material that has attracted overwhelming attention, was
discovered9,10,11,12. Even if very little is known concerning
their liquid phase growth mechanism or their structural and photo-physical properties,
elongated lead halide perovskite particles9,13 have already been
successfully integrated into perovskite-based solar cells14 and
ultrasensitive, micro-fabricated photodetectors15. Recently, exceptional
lasing properties of solution-grown perovskite thin films16,17; and
nanowires18 have been reported.

Solution-based low-temperature processes are generally recognized to be cost-effective
and easy to scale-up fabrication methods. Therefore, the forecasted low fabrication cost
is one of the main arguments of researchers and investors for considering organometallic
halide perovskites as a promising material for the development of next-generation
photovoltaics19. Despite numerous successful reports on
record-breaking efficiencies it remains a major challenge to attain device-to-device
reproducibility of the performance parameters, even in case of lab-scale
(~20 mm2 surface area) prototypes20. Currently, the major hurdle to overcome is related to the lack of
precise control of the crystal parameters, such as the crystal habit, crystallite size,
crystallinity and the quality of grain boundaries on small and large scale surfaces. For
instance, in polycrystalline nanoparticle-based perovskite films, the most important
parameters to achieve outstanding optoelectronic performances are strongly linked to
both the size and the orientation of the crystalline domains, likewise affecting the
film homogeneity, area coverage, pinholes and roughness21. Similarly, the
slip-coating method, i.e. our previously reported single-step approach, produces
perovskite nanowires in dandelion-like organization on a flat surface.

The liquid phase growth pattern is determined by the formation of nucleation centers
randomly distributed over the substrate, therefore there is little control on the area
coverage, pinholes, aspect ratio and orientation of the nanowires9,15.
Recently, however, we found a fairly simple approach to overcome the spatially-random
surface nucleation. Therefore, here, we report on the guided growth of extremely high
aspect-ratio perovskite crystalline nanowires in the arrays of open nanofluidic
channels. An important feature of the observed
“solvatomorph-graphoepitaxy” is that the crystallization of
MAPbI3 solvatomorphs proceeds exclusively in polar aprotic solvents such
as dimethylformamide (DMF), dimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO), at
near room temperature. Moreover, for the first time, the kinetics of the nanowire growth
was visualized with a simple optical microscope in real-time. We envision that
standardization of this general strategy will allow the reproducible fabrication of
wafer-scale perovskite nanowire thin films with highly controlled crystallite dimensions
and crystallinity. Their integration into various optoelectronic devices could help in
further boosting their performances.

We show that slip-coating of saturated solution of MAPbI3 in polar aprotic
solvents (i.e. DMF, DMSO and DMAc) results in the formation of perovskite nanowires
organized in dandelion-like arrangement on a silicon surface. As can be seen from the
optical microscopy and scanning electron microscopy (SEM) images in Fig.
1, the liquid phase growth pattern is inherently determined by the formation
of nucleation centers, which are randomly distributed on the surface. Therefore, there
is little control over the aspect ratio and the orientation of the nanowires. Only a
very slight, shear-force-induced guidance was observed with respect to the sliding
direction of the top glass plate.

Figure 1: Schematic illustration of the slip-coating process

(a–c) and optical images extracted from a video of the growth of a set of slip-coated MAPbI3 nanowires (d–f). SEM micrographs of a representative set of nanowires (g). AFM distribution of length and width of the set of nanowires shown in g. The liquid phase growth pattern is inherently determined by the formation of nucleation centers randomly distributed on the surface. The nanowires grow in dandelion-like pattern. The nanowires grow in radial direction out from the nucleation centers and form an assembly of crystallites that resembles a “paper fan”. We do see wire-wire intergrowth but we do not observe the formation of secondary bunches. The slip-coating process has no precise control over the dimensions of the as-synthesized nanowires.

MAPbI3 has been reported to be unstable in the majority of ordinary solvents,
including water, and to decompose at moderate temperatures
(~400 K) in addition22,23. Therefore, processing
and precise patterning by post-growth assembly techniques such as dielectrophoresis24, Langmuir-Blodgett self-assembly25 and mechanical
shear1 seems to be a challenging task. Unlike the above mentioned
multi-step post-growth assembly techniques, here, we combine a bottom-up nanofluidic
alignment with a top-down surface patterning technique to achieve both the synthesis and
oriented assembly of elongated MAPbI3 crystallites in a single step. The
nanowires were crystallized in the arrays of nanofabricated channels owing to the strong
guiding effect of the nano-grooves26. Similar guiding effects have
already been observed for chemical vapor deposited (vapor-liquid-solid grown, VLS) grown
gallium nitride (GaN) nanowires27. The phenomenon of the alignment
induced by the intimate epitaxial relationship has been discovered four decades ago and
it has been named “graphoepitaxy”28,29,30. In a
typical experiment, we dropped a supersaturated solution of MAPbI3 dissolved
in DMF onto the arrays of nanostructured trenches etched in a SiO2 substrate
(Fig. 2a). Next, capillary forces drove the liquid inside the
channels with a speed proportional to the channel width31 (Fig. 2b). At this point, the first defects and etching-induced
crystallographic imperfections of the channel wall triggered the heterogeneous
nucleation of a yellow, so far barely studied clathrate phase of MAPbI3-DMF.
This crystalline, translucent-yellow precipitate contained the mother liquor, that is a
polar aprotic solvent (e.g. DMF, DMSO and DMAc). Therefore, it can be regarded as
a solvatomorph phase (i.e. metastable precursor phase) of the MAPbI3
perovskite. Unlike MAPbI3 itself, this solvatomorph phase does not show the
characteristic photo-luminescence under excitation with the green monochromatic
incoherent light
(λex = 546 nm) (Fig. S4). Interestingly, neither the
solvatomorph formation nor the guided growth was observed with gamma-Butyrolactone
(GBL), another commonly used solvent in solution based perovskite thin films
preparation. This partially explains the enigma of the highly-anisotropic
crystallization of cubic or tetragonal phases of the perovskite. Actually, the
anisotropic crystal growth is specific to the metastable translucent-yellow colored
solvatomorph phase, formed by the host-guest interaction (solvatomorph formation) of
MAPbI3 with polar aprotic solvents (MAPbI3-DMF,
MAPbI3-DMSO and MAPbI3-DMAc). It is important to realize, that
these are metastable precursor compounds, hence the final MAPbI3 perovskite
phase is formed by a subsequent solvent evaporation-induced recrystallization of the
solvatomorph phase, which is equivalent to the dehydration processes observed for
instance in many oxo-hydroxo compounds (Fig. 2g,h). The analysis
of the time-lapse videos recorded in an optical microscope provided insight into the
nanowire formation mechanism and also added another simple tool to study the kinetics of
growth and dissolution (Fig. 2, Fig. S6, video
S1). We have found, that the crystallization follows the classical solvent
evaporation-induced supersaturation-driven crystallization. The most important
parameters controlling the growth rate are the surface-normalized concentration of the
MAPbI3 solution, as well as the temperature and the surface tension of
the solvent. In the first approximation, stable clusters and nuclei of
MAPbI3-DMF solvatomorph form in the channel entrances, thus acting as foreign
particles (Fig. 2e). The subsequent nanowire growth, dilutes the
liquid, hence shifts the solution to an undersaturated condition (Fig.
2f). This is expected to slow down the kinetics and ultimately stop the
precipitation of the solute. However, the solvent evaporation from the open nanofluidic
channel acts on the opposite way, it concentrates the solution, maintaining the
supersaturation condition in equilibrium (Fig. 2g). Furthermore,
the capillary forces as well as the concentration difference-induced diffusion of the
solute toward the growing crystal plane, i.e. the growing end of the wire, play a
fundamental role in crystal growth. When the continuous supply of the solute and/or the
mother liquor is blocked the supersaturation is exhausted, the nanowire growth stops
(Fig. 2c). The synthesis process ends when all the mother
liquor escapes from the metastable clathrate phase (MAPbI3-DMF) formed by the
polar aprotic solvent. Nanoscale crystallite dimensions allow the solvent to escape
without inducing significant shrinkage-induced cracks or damage in the crystal habit.
Ultimately, the precursor phase transforms to the final, grey-silver MAPbI3
phase and maintaining the elongated crystal shape (Fig. 2d).

Figure 2: Snapshots of a video showing the graphoepitaxail growth process in a dense array of 500 nm wide channels etched in Si

(a). False-color SEM micrographs of the growth process implemented in nanofluidic channels realized with a high resolution positive e-beam resist, ZEP520A (b–d). 200 nm-wide array of ZEP520A nanochannels on SiO2 substrate (b) MAPbI3nanowires synthesized in the nanofluidic channels (c) MAPbI3 nanowires after the removal of ZEP520A resist by chloroform (d). Conceptual illustration of the aligned growth of MAPbI3 nanowires in nanofluidic channels (e–h) The saturated MAPbI3 solution is drop-casted on a series of open nanofluidic channels realized by ZEP520A (e). The solution is driven by capillary forces inside the channels. The nucleation takes place at the first defect present in the channel and the growth process starts (f). When the solution supply is stopped, the system does not fulfil the growth conditions anymore and synthesis stops (g). After all the solvent has evaporated, the nanowires transform from their metastable phase to MAPbI3(h). Detailed structural and photophysical characterization of MAPbI3 nanowires have been reported in our previous work.9,15

The process yields nanowires with well-defined sizes, cross-sections and spatial
distributions. With this fairly simple technique, extremely long (up to few mm) and
narrow (down to 10 nm) V-shaped (Fig.
S2a) and rectangular-shaped cross section (Fig. S2b) nanowires have been realized along
predefined nanofluidic channels. Examination of the growth process on larger length
scales shows that the growth extends over millimeter length scales and seems to be
limited by the length of the fluidic channel. We have carried out several experiments in
order to understand the parameters controlling the crystal habit.

We have found that whereas wider, micrometer-sized channels were mostly filled with
bundles of nanowires (Fig. S2d), individual
nanowires tend to grow in narrower channels. This can be explained by the stochastic
nature of the heterogeneous nucleation process. Narrow channels are comparable in size
to the first nucleus formed, hence there is a higher probability that they contain less
nucleation centers, resulting in individual elongated single crystallites (Fig. S1). In contrast, wider channels
(>1 μm) contain more imperfections, where multiple nuclei
can be formed, resulting in the formation of crystallographically-fused parallel
aggregates of perovskite nanowires (Fig.
S2d, Video S4). The crystal growth of
the elongated solvatomorphs is likewise feasible in curved or zig-zag shaped nanofluidic
channels (Fig. 3a). The graphoepitaxial growth is not limited to a
particular inorganic substrate, hence the guided growth was similarly observed in
metallic (e.g Au) as well as in organic resin (e.g. ZEP520A) based nanofluidic channels.
This compositional versatility enables the fabrication of numerous patterned
substrate/nanowire material combinations i.e. p-i-n lateral interdigital configurations
(Fig. 3). The possible advantage of these non-conventional
p-i-n architectures over the state-of-the-art planar or mesoscopic configurations
is yet to be determined (Fig. S7).
Interestingly, we observed guiding effects even in between non- continuous -walled
channels, e.g. arrays of rectangular pillars of SiO2 or organic resin of
ZEP520A (Fig. 3d,e). This implicates that the most important
parameter determining the nanowire diameter might be the size of the nucleation center.
Once the nanowire growth starts, the cross-section remains constant over ~cm
length scales (Fig. 3g, Video S3, Video S4). This allowed
us to create controlled ‘wire-to-wire’ connections by launching
the wire growth on the pillar-pattered silicon surface from two directions
(90o degree angle) simultaneously (Fig. 3e). In
addition, the surface coverage and the film thickness can be similarly controlled by the
nanofluidic channel dimensions. Importantly, all the wires were spatially confined and
the minimum separation of the aligned nanowires depends solely on the resolution and
precision of the applied lithographical process. By using recent electron-beam tools,
this can easily be reduced below 50 nm. Accordingly, the channel width and
its periodicity will control the area coverage of a given film, with the film thickness
being linearly proportional to the channel height.

Figure 3: Colored SEM micrographs of different set of MAPbI3nanowires.

Curved nanowires after ZEP520A removal (a) dense array of aligned MAPbI3 nanoparticles (b) nanowires over etched patterns (c) perovskite nanowires grown with the guidance of Si pillars (d) that can be used to obtain cross-bar architectures; the growth directions are marked with arrows (e) Optical image of mm2-sized surfaces composed of MAPbI3nanowires with different widths and spacings illuminated with white light. The blue, green, orange and red colors are due to the interference pattern of millimeter long periodic nanowire arrays (f and Video S2); extremely long, 200 nm-wide array of MAPbI3 nanowires on SiO2 after ZEP520A removal (g).

As an example, we fabricated series of perovskite nanowire based photodetector devices
using e-beam lithography (Fig. 4e). The longitudinal composite
devices were composed of lithographically patterned graphene sensitized by a perovskite
nanowire active layer (Fig. 4a,b). Recently we have reported on
the fabrication of similar devices attaining remarkable responsivities
(2.6 × 106
AW−1) under pW light intensities. Here, the hybrid
devices prepared by the graphoepitaxial liquid-solid growth of aligned, millimeter-long
nanowires of MAPbI3 in open nanofluidic channels showed similar linear
I-V characteristics, with characteristic response times of 2–5
seconds (Fig. 4d). The best devices reached responsivities as high
as 6 × 106
AW−1 (Fig. 4e). The drastic
enhancement of the responsivity at very low light intensities (pW) could enable
MAPbI3 nanowire/graphene devices for use as low-light imaging sensors and
single photon detectors. We attribute these very high device performances mainly to the
controlled growth of perovskite nanowires in predefined positions. Our results
demonstrate an important step in the integration of perovskite nanowires in arrays of
microfabricated optoelectronic devices with high reproducibility. The process allows the
synthesis of extremely long (~cm) and thin (~few nm) nanowires
with a morphology defined by the shape of nanostructured open fluidic channels.
Moreover, optimized spacing, shape and size in the periodic pattern may result in
large-area, wafer-scale superstructures with advantageous optical properties for
photonic, optoelectronic, radio-frequency and non-linear optic applications. Ultimately,
the graphoepitaxial nanowire growth enables well-engineered angular restriction designs,
which can improve the photon reabsorption process and thus maximize the performance of
perovskite nanowire based optoelectronic device32. This low temperature
solution growth method, unique in its gender opens an entirely new spectrum of
architectural design of organometal halide perovskite based optoelectronic devices.

The nanowires were grown in 250 nm wide ZEP520A channels. The ZEP520A resist was dissolved prior the SEM imaging (a). Diagram of the device architecture (b) I-V curve of the best performing MAPbI3 nanowire sensitized graphene photodetector illuminated with pW light intensities (c) Time response of a representative device (d). Two regimes can be identified: a fast one corresponding to ~70% decay (~2–5 s) and a slow one of ~10–20 s associated with the charge traps in nanowire film. Responsivity of the device for different incident light intensities (e).

Acknowledgements

This work was partially supported by the Swiss National Science Foundation and ERC
ADVANCED GRANT (PICOPROP#670918). Device fabrication was carried out in part in the
EPFL Center for Micro/Nanotechnology (CMI).

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Contributions

L.F. initiated the research. E.H. synthesized the perovskite solutions and discovered the graphoepitaxial nanowire growth. M.S. and E.B. prepared the microfabricated devices. M.S., E.B. and B.N. performed the photocurrent measurements and analyzed the data. A.S. performed and analyzed the fluorescence measurements. E.H., A.S., M.S., E.B., B.N. and L.F. discussed the results and implications and commented on the manuscript at all stages.

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